1. Field of the Invention
This invention relates to a method and an apparatus for producing sintered iron ores with a downward air suction flow type sintering machine such as a DL (Dwight-Lloyd) type sintering machine, a GW (Greenawalt) type sintering machine, etc.
2. Prior Art
In a DL sintering process, a sintering reaction proceeds while air is drawn downwardly through the sintering bed and coke contained in the raw materials in the sintering bed is combusted, thereby moving a combustion-melting zone having a thickness of a few mm to a few tens of mm downwardly in the thickness direction of the raw materials in the sintering bed on the pallet, as disclosed in Tekko Binran (Iron & Steel Handbook) II, Seisen Seiko (Pig Iron & Steel Making), third edition, page 106 et seq., compiled by Nihon Tekko Kyokai (Association of Iron and Steel of Japan) and published on Oct. 15, 1979.
In the DL sintering process, the sintering proceeds along with the combustion-melting zone of the sintering bed with the air being drawn through the already sintered cakes in the upper level region being preheated. Thus, the raw materials are liable to undergo sintering in a heat excess state in the combustion-melting zone, whereas the raw materials are liable to undergo sintering in a heat deficient state in the upper level region. Thus, an amount of molten material in the combustion-melting zone is increased in accordance with a heat gradient in the thickness of the layer of raw materials.
In addition, the sintered cakes in the upper level region press downwardly against the combustion-melting zone due to the weight of the formed sintered cakes and also due to a downward force on the sintered cakes created by the suction of a blower. The molten materials under such load are highly liable to clog the pores in the sintering bed, and the necessary permeability conditions for stable combustion of the coke breeze contained in the layer of raw materials are deteriorated in the combustion-melting zone of the sintering bed, resulting in a decrease in the sintering speed. Simultaneously, a lower yield and increased NOx results due to the deterioration of coke breeze combustion. Besides, qualitatively the strength of the sintered ores is lowered and the number of pores is reduced, resulting in poor reducibility.
To solve these problems, it was proposed to improve the permeability by decreasing the layer thickness to reduce the permeability resistance or by using an increased amount of quick lime to intensify granulation of raw materials. However, the former results in lower yield and strength and the latter requires the use of expensive quick lime.
Japanese Patent Application Kokai (Laid-open) No. 2-254125 discloses an effective method for preventing reductions in the yield and quality at a low cost by supporting sintered cakes by the use of stand materials. However, the proposed method still has problems in that they require the periodic replacement of the stand materials used for supporting the sintered cakes, due to abrasion of the stand materials.